The properties of a toner are set through the selection of materials and amounts of the materials of the toner. The charging characteristics of a toner are also dependent on the carrier used in a developer composition, such as, the carrier coating. Toners typically comprise one or more external surface additives, for example, silica, titanium dioxide, zinc stearate and the like. The carrier coating can interact with a toner surface additive.
Toners having a triboelectric charging property within the range of about −30 μC/g and about −45 μC/g may be achieved when using smaller-sized silica particles as external additives, for example, silica particles having average sizes less than 20 nm, such as, for example, R805 (˜12 nm) and/or R972 (˜16 nm) (Evonik, N.J.). However, developability at areas of low toner area coverage degrades over time. That has been attributed to the small-sized additives being impacted into the toner surface over time.
The problem with smaller-sized additives has been addressed by using larger-sized additives, i.e., additives having a size of 40 nm or larger such as, for example, RX50 silica, RX515H silica or SMT5103 titania (Evonik, N.J.). However, although the above developability problem is addressed, the toners do not exhibit as high a triboelectric charging ability and also exhibit charge through.
New carrier coatings are being developed that enable higher charging developers, particularly those with larger-sized additive package components. However, when such developers are tested at low area coverage followed by high area coverage, the developers tend to exhibit low or wrong sign toner due to charge through, i.e., the incumbent toner in the device becomes less negative or even wrong sign, i.e., positive, and the new (fresh) toner added may charge very negative. The presence of low charge and/or wrong sign toner can result in objectionable background.
There remain problems with providing high charge with good relative humidity (RH) sensitivity of charge to changing environmental conditions for carrier coating resin designs. For example, there remains a requirement to be able to tune the charge of the carrier resin to produce higher charge for those situations where higher charge is required. To obtain both high charge and good RH sensitivity through a single design has been challenging.
Further, toners containing silica as a surface additive, which silica can be a charge driver for the toner, are known to be very RH sensitive. Thus, it is important to provide new carrier resin designs that work well with silica to improve RH sensitivity, while maintaining high charge.